The term “genlock” as used in the television industry, represents an abbreviation for the terms “generator locking” and typically refers to synchronizing a video signal to a clock signal of a prescribed frequency. Most if not all video cameras and other sources of video signals have a local oscillator for locking the video signal generated by the device to the local oscillator frequency. However, the local oscillator frequency of one source will not necessarily have the same phase as the local oscillator frequency of another source, even if both local oscillators have same frequency. Such a phase difference can adversely affect the processing of such signals. To achieve synchronism between video sources, a synchronizing (sync) generator provides a common genlock (sync) signal to each video source.
To understand the process of synchronization, some background on video signals will prove helpful. The horizontal blanking interval of an NTSC or PAL composite video waveform contains horizontal synchronization (H-sync) portion and a color sub-carrier burst signal component. The color sub-carrier bust component has 9 or 10 sub carrier cycles, depending on whether the video signal is NTSC or PAL, respectively. A synchronizing signal, typically in the form of a 27 MHz signal generated by a Voltage Controlled Oscillator (VCXO), locks to either the H-sync portion or the burst component of the composite video reference signal. Locking the synchronizing signal to the burst component provides a more stable sync signal (i.e., reduced jitter) as compared to locking to the horizontal sync portion since much more signal “information” resides in the 9 (NTSC) or 10 (PAL) burst sub-carrier cycles than in the falling edge of the H-sync signal. Additionally, locking to the burst component yields a sync signal much less influenced by noise residing on the reference video signal, as compared to locking to the H-sync portion.
Analog sync generators that lock the 27 MHz signal of the VCXO to the burst component of the video signal generally offer superior jitter and noise handling performance. However, the implementation of an analog sync generator requires a large number of commercially available analog components and extensive calibration to guarantee repeatable performance. In addition, color-frame sequencing is difficult to implement in an analog sync generator. In this regard, burst-locked loops utilized in present day analog sync generators typically require more design effort as compared than sync-lock loops, particularly due to the frequency relationship between the color sub carrier frequency and the 27 MHz clock signal. The ratio of the Frequency clock (Fclock) to the Sub-carrier Frequency (Fsubcarrier) for a NTSC video signal is given by Fclock/Fsubcarrier=35/264 while for a PAL video signal, the ratio Fclock/Fsubcarrier is 709379/4320000. For sync locking, the ratios are much more simple, yielding a ratio of Fclock/Fsync=1/1716 for a NTSC video signal and ratio of Fclock/Fsync=1/1728 for a PAL video signal.
Digital sync generators typically synchronize the 27 MHz of the VCXO to the horizontal sync portion of the incoming video signal. As compared to analog sync generators that synchronize the 27 MHz signal of the VCXO to the burst component of the video signal, present day digital sync generators generally offer lower cost implementations. However, the jitter and noise handling performance of present digital sync generators make them inferior to analog sync generators.
Thus, there is need for a digital synchronizing generator that offers comparable performance to analog genlock techniques, while offering reduced complexity and cost.